The present invention relates to a penetration meter i.e. a device for performing underground exploration, and intended particularly, but not exclusively, for exploring the sea bed, at depths that may be as great as several tens of meters.
One of the means commonly used for evaluating the mechanical characteristics of terrain that is to constitute the foundation of civil engineering work, is to perform static penetration testing, known as CPT.
The test consists of pushing a rod into the terrain concerned, which rod is fitted at its bottom end with sensors serving, throughout penetration, to measure continuously various parameters such as tip resistance and lateral friction.
These measurements are transmitted via a cable passing along a central bore in the rod. Conventionally, the rod is cylindrical and has a diameter of 36 mm. Penetration is performed at a continuous speed of 2 cm/sec. Investigation depth, and thus rod length, vary depending on circumstances over the range several meters to several tens of meters. The thrust force on the rod which is a function of the resistance of the terrain and of the depth of the investigation, commonly lies in the range several metric tons to 20 tons, above which value testing is normally stopped.
Thrust is generally exerted by means of an actuator acting on the top end of the rod. In practice, the rod is made up of successive elements that are screwed to one another. The length of each element does not exceed a few meters for practical reasons of actuator length and ability to withstand buckling under thrust force. Penetration is performed in steps, each step corresponding to the length of one element pushed by the actuator.
When investigating a site on land, the operations of connecting the elements end to end and of passing the measurement cable along the rod require manual operations to be performed, thereby presenting the drawback of significantly lengthening the time required for operation.
When investigating the sea bed, it is practically impossible to use manual intervention, so other modes of operation have been developed in order to eliminate the problem of connecting a string of rod elements together end to end. The one-piece rod, i.e. having a length that may be several tens of meters, is no longer thrust by applying an actuator to its end, but by means of a continuous thrust system using wheels. The two horizontal-axis wheels face each other and clamp the rod in facing grooves. Rotating the wheels makes it possible to apply a driving force. The drawback of such a system is associated with the clamping force that must be exerted by the wheels on the rod to develop the driving force. Given the mechanical characteristics of penetration meter rods, that system cannot be used for developing more than a few tons of thrust per pair of wheels. To reach the commonly-required twenty or more tons, several pairs of wheels must be stacked above one another.
A major drawback of that system results from difficulties associated with handling and using rods that are several tens of meters long, particularly for use in deep water.
To mitigate that drawback, proposals have been made to use the xe2x80x9ccoil tubingxe2x80x9d technique which consists in winding the tube on a reel of large diameter. During the winding operation, the tube is subjected to plastic deformation. When performing the CPT testing, the tube is unwound progressively and is straightened out by a set of presser wheels, thereby subjecting it again to plastic deformation.
Such a system makes it possible to reduce problems associated with the space occupied by a one-piece tube, however the work-hardening to which the penetration meter rod is subjected during successive operations of being wound up and of being paid out and thrust into the ground, limits application to rods of small diameter for investigating shallow depths using forces of a few tons only, and in any event leads to the rapid deterioration of the tubes.
An object of the present invention is to mitigate those drawbacks and to make it possible to perform CPT type testing capable of penetrating to several tens of meters, using compact equipment that can be implemented automatically and that does not require manual intervention while testing is taking place.
According to the invention, the device for exploring the subsoil comprises a tower, mutually engageable rods, and means for applying pressure to a top end of the top rod of a string of rods, and the rods are hinged to one another and are disposed on a drum, the drum having a core of square section and of side equal to the length of a rod, the drum being carried by a bracket secured to a slider that is vertically movable in fixed guides, the drum carrying in the vicinity of its middle portion, a pair of claws or clamps that are retractable and that co-operate in succession with different ones of the corners of the square constituting the drum.
The string of rods is a continuous string, with the rods being hinged to one another. The string becomes rigid as the rods are brought successively into alignment. On the drum, the rods preferably rest side by side. Thus, pressure is always applied axially, thereby avoiding buckling, and secondly, guides are provided at the bottom portion of the tower.
Operation is as follows: after the tool has been fixed on one end of the first rod, the first rod is disposed vertically. The actuator presses on the second end thereof, thereby causing the tool to penetrate into the ground down to a depth equal to the length of the rod. Thereafter, the rod of the actuator is raised, thereby raising the bracket and likewise raising the drum. However the retractable claws situated beside the tower hold the top corner of the drum. As a result the drum rotates counterclockwise while it is being raised. This rotation through one-fourth of a turn brings the next rod which was previously in a horizontal position into a vertical position. During the next cycle, the clamps are retracted, the actuator lowers the drum, and the first rod together with the tool is pushed down into the ground by the second rod through a length equal to the length of one rod. These operations are continued until the desired depth has been reached.
The tool is raised in the opposite order, the clamps then preventing the corner that is at the bottom of the drum from rotating so as to prevent it from moving down, thereby obliging it to turn clockwise. The rods are thus put into place next to one another on the drum as they are raised.
It will be observed that no large force is exerted on the axis of the drum which need only support the weight of the drum plus the weight of the rods wound thereabout. Drive from the actuator is applied only to the rods, and only to the ends thereof.